Porous membranes are used as separation membranes in various industrial fields. For example, the membranes are widely used for the preparation of ultrapure water in the field of semiconductor production, the removal of a very small amount of iron contained in cooling water at power plants and filtration or the removal of microorganisms in medical appliances and in the pharmaceutical and food industries. The current trend is one of continuing expansion in the range and volume of the application and use of such membranes. Above all, the demand for porous membranes excellent in heat and chemical resistance is on the increase. For example, in the semiconductor production field, it is desirable to raise the temperature of ultrapure water in order to effectively wash the fine regions of semiconductors with the water. Therefore, porous membranes are in demand from which ion fractions, organic substances or the like are eluted in small quantities at high temperatures and which are excellent in heat and chemical resistance. At thermal or nuclear power plants, separation membranes are in demand which have particularly excellent heat resistance and which make it possible to stably remove cruds (particulate substances mostly comprising iron) from steam condensates having a temperature of 100 degrees C. or higher for a long period without cooling the steam condensates.
At present, materials used for porous membranes include cellulose derivatives such as cellulose acetate, etc. and polymers such as polyacrylonitrile resins, polyamide resins, polymethyl methacrylate resins, polysulfone resins, polyvinylidene floride resins, polyethylene resins, polycarbonate resins, etc., and membranes prepared from these materials are widely employed for ultrafiltration or microfiltration. However, the membranes of these materials are inferior in heat, chemical and boiling water resistance, so that the membranes are not suitable for the aforesaid uses. For the above-mentioned reasons, attention has been paid to aromatic polyether ketones as materials for filtration membranes because of their extreme excellence in heat and chemical resistance. Therefore, attempts have been made to develop porous membranes made from aromatic polyether ketones.
For example, JP-A-3-21333 (corresponding to EP-A-8894) and JP-B-6-34912 propose separation membranes comprising sulfones of polyether ether ketones which are aromatic polyether ketones and processes for preparing the membranes. Membranes of such sulfones, however, are known to swell in water (Macromolecules, 86, p.18, 1985). Further, the membranes dissolve, intensely swell or deform in organic solvents such as acetone, alcohols, tetrahydrofuran, dimethyl formamide, so that their application range is limited. The proposed membranes, therefore, cannot be employed in fields that require the high performance of filtration membranes.
Furthermore, porous non-sulfonated aromatic polyether ketone membranes from melt or wet processes and the production processes thereof have been proposed.
Referring to the melt processes, for example, JP-B-5-33267 (corresponding to U.S. Pat. No. 4,755,540) and JP-A-3-106424 (corresponding to EP-A-417908) propose processes for preparing porous membranes useful as separation membranes wherein aromatic polyether ketones, thermoplastic polymers which are imcompatible with said aromatic polyether ketones, and plasticizers are melt compounded under heat at high temperatures to obtain mixtures, which are formed into desirable shapes, cooled and washed to remove said imcompatible thermoplastic polymers and plasticizers.
JP-A-3-237142 (corresponding to EP-A-409496) and JP-A-4-293533 (corresponding to U.S. Pat. No. 5,227,101) propose processes for preparing porous membranes in the same way as mentioned above except that compatible plasticizers and latent solvents are used as plasticizers. However, according to these processes it is difficult to prepare membranes which are anisotropic in cross section, although the membranes have open pores on the surfaces thereof. Even if some membranes mentioned above are anisotropic in structure, the membranes do not have an excellent balance of water permeability and fractionating characteristics since one of their surfaces has no open pores or has a tight skin layer with an open pore ratio of 7 percent or lower. Particularly, the membranes prepared according to the latter processes have a problem of extremely low water permeability. Further, there is another problem in that the imcompatible thermoplastic polymers and plasticizers used in the preparation of the membranes tend to remain therein and to be eluted therefrom when they are used as separation membranes.
JP-A-7-776 proposes a process for preparing a membrane comprising an aromatic polyether ketone and inorganic or organic particulates, wherein said membrane is prepared by melt compounding the aromatic polyether ketone and the particulates to obtain a molten mixture, which is formed into a desirable shape, cooled and stretched. However, in general the membrane that is obtained according to the above-mentioned process tends not to be uniform in pore size on the surfaces thereof. Further, when the membrane is used for filtration of water at high temperatures for purification purposes, the particulates and ion fractions which are contained in the membrane tend to be eluted therefrom into the hot water filtrate.
Referring to the wet processes on the other hand, JP-A-2-136229 (corresponding to U.S. Pat. No. 4,992.485), JP-A-3-56129 (corresponding to EP-A-382356) and JP-A-3-174231 (corresponding to U.S. Pat. No. 5,082,565) propose processes for preparing porous membranes, wherein aromatic polyether ketones are homogeneously dissolved in strong acids to prepare membrane forming stock solutions, which are formed into desirable shapes and immersed in poor solvents against the polyether ketones for the precipitation and coagulation thereof. JP-A-3-172349 (corresponding to U.S. Pat. No. 4,897,307) proposes a method of crystallizing the thus obtained membranes. These wet processes are effective for obtaining membranes which are asymmetric in cross section, but the surfaces of these membranes do not have open pores that can be observed with a scanning electron microscope or have tight skin layers which are very low in open pore ratio. For these reasons, the membranes are useful for ultrafiltration but are unusable, for example, for microfiltration because of low water permeability. Further, the membranes proposed in the former patent publications are low in heat resistance because the membranes are not heat stabilized.